U-Bend Radius and
Tube Bending

U-Tube Heat Exchangers For

U-Bend Radius and Tube Bending
U-Tube Duty

Bending a tube through 180 degrees stretches the outside and compresses the inside. Both effects are permanent, both are predictable, and both have to be designed for rather than discovered.

The outside of a bend travels further than the inside, so material there stretches and the wall thins. The inside compresses and may thicken slightly. The tighter the radius relative to tube diameter, the more pronounced both effects become.

Minimum radius. A centerline radius of at least twice the tube outside diameter is the general working rule. Below that, thinning becomes significant enough that the tube gauge has to increase, which costs money and reduces the inside diameter available for flow.

Why the Bend Is Where Failures Start. Three things coincide there. The wall is thinnest, the residual stress is highest, and the material is hardest. Any corrosion mechanism that needs tensile stress finds ideal conditions.

In chloride service on 300-series stainless, and in sour service on carbon and low alloy steels, the bend is the location worth inspecting first. Eddy current testing covers it, and it is worth including the bend region specifically in an inspection scope rather than sampling straight lengths.

Radius and Alloy Are One Decision. Duplex grades work-harden noticeably during bending and need a more generous radius than austenitic stainless. High-strength and high-nickel alloys have their own formability limits. Titanium bends well but springs back differently.

Choosing the alloy first and the bend radius second produces designs that have to be revisited. Send the service conditions and let both be decided together.